1. Field of the Invention
The present invention relates to a multi-layered printed-coil substrate for use as planar magnetic components, wherein the multi-layered printed-coil substrate includes a single or a plurality of substrates which has patterned coils.
2. Description of the Background Art
Wound magnetic components are known in the art and in common use as transformers and choke coils used in the switched mode power supply circuits and the like. The known wound wag tic component is composed of a bobbin having lead terminals, the bobbin being wound with an enamel wire or the like. This type of magnetic components are advantageous in that the number of turns and turn ratios can be readily changed so as to obtain an optimum transformer ratio, thereby facilitating the designing and developing of circuits, especially the manufacturing of transformers having an optimum transformer ratio.
In general, the industry is in a strong need for reduction in the size and weight of electronic devices, and such demands are reflected in the minimizing of circuit components. As one of the proposals for meeting such demands, planar magnetic components have been developed instead of the conventional wound magnetic components. Examples of planar magnetic components are disclosed in Japanese Patent Publication Nos. 39-6921, 41-10524, and Laid-Open Publication No. 48-51250. The planar magnetic component is not fabricated by winding a wire into a coil but, for example, a flat insulating substrate is used on which a conductive pattern is formed with a thin film in a letter-U form or a spiral form. In this way a printed-coil substrate is obtained. A single substrate or several substrates are layered into a unit which is then sandwiched between magnetic cores. However, the number of turns is limited because of the restricted space on the substrate. To overcome this limitation, it is required that several printed-coil substrates are layered into a single unit.
Planar magnetic components are advantageous in that the size and height can be minimized, and the leakage inductance is minimized because of an increased area for interlinkage of the magnetic flux thereby to strengthen coupling between the primary and secondary windings, and the minimized copper loss due to skin effect. In addition, the coil is formed by etching which is more stable than the wire winding, thereby enhancing productivity and maintaining quality control. Among these advantages the high coupling between the primary and secondary windings and the restraint of copper loss will be more appreciated when the components are used under a high frequency current. In the field of switched mode power supply circuit where the use of high frequency current is becoming more and more popular, planar magnetic components call the industry's attention.
FIG. 1 shows examples disclosed in Japanese Patent Laid-Open Publications Nos. 61-74311 and 61-75510, for example. A wiring substrate 41 is composed of layered insulating sheets each having coil patterns 45 formed thereon. The wiring substrate 41 as a whole constitutes a multi-layered printed-coil substrate used for a transformer. The wiring substrate 41 is provided with through-holes 42 through which terminals 43 in the form of pins (hereinafter "pin terminals") are inserted and soldered thereto, thereby ensuring that the coil patterns 45 on one substrate and another are electrically connected. One end of each pin terminal 43 is extended as shown in FIG. 1C and used as a connector to an external conductor (not shown). The wiring substrate 41 is sandwiched between a pair of split cores 44 and 46. In this way a magnetic circuit is completed in the transformer.
FIG. 2 shows another example of planar magnetic component which is disclosed in Japanese Utility Model Laid-Open Publication No. 4-103612. A coil pattern 52 is formed in a spiral form on a wiring substrate 51. The wiring substrate 51 is provided with three apertures 53, 54 and 55. A pair of ferrite cores 56 and 57 are prepared; the core 56 is provided with three projections adapted for insertion through the apertures 53, 54 and 55 of the wiring substrate 51. The core 57 is provided with recesses for receiving the projections of the core 56. In this way a magnetic circuit for transformers is formed.
FIGS. 3 and 4 show further examples which are disclosed in Japanese Utility Model Laid-Open Publication No. 4-105512, Patent Laid-Open Publications Nos. 5-291062 and 6-163266. The illustrated thin-type transformer includes a multi-layered printed-coil substrate 62 placed on a base 63 which is provided with pin terminals 65 each of which includes a vertically extending portion 65a and a horizontally extending portion 65b. The vertically extending portions 65a are inserted through through-holes 66 in the multi-layered printed-coil substrate 62 and soldered thereto so as to effect electrical connection. The multi-layered printed-coil substrate 62 is sandwiched between an I-shaped core 64 and an E-shaped core 61, thereby forming a complete planar magnetic component. The finished component is connected to an external conductor through the horizontally projecting portions 65b.
The known planar magnetic components have advantages pointed out above, but on the other hand, they inherently have the difficulty of changing the number of turns and ratios of winding, and when these changes are wanted, a fresh printed-coil substrate must be fabricated after a new coil pattern is designed. This involves a time- and money-consuming work. Eventually, the components must be used where the number of turns and ratio of winding are fixed. The advantages inherent in planar magnetic component are not fully utilized.
The example shown in FIG. 1 has difficulty in enabling the pin terminals 43 to align with the through-holes 42 and vertically position therein. This aligning work is time-consuming, which is reflected in the production cost.
As far as the aligning is concerned, the examples of FIGS. 3 and 4 are more advantageous than the example of FIG. 1 because of using the base 63 having pin terminals 65 uprightly fixed in alignment with the through-holes 66. The use of the base 63 can reduce the number of producing steps. On the other hand, the complicated base 63 is costly, so that the whole production cost cannot be reduced. For the purpose of mass-production, one way is to standardize the base 63 in the shape (the size, the pin terminal pitches, the number of pin terminals) but this is contradictory to users' demand. Users want to have a variety of bases even in a small quantity in accordance with required magnetic characteristics. If the bases are standardized in one or two fixed models, the range of applications will be restricted. The examples of FIGS. 3 and 4 lack the freedom of designing the configuration of bases, and there is no choice but to use expensive bases 63.
In the example shown in FIG. 2 the coil pattern and the external conductor are constituted on the same substrate, thereby requiring no terminal base or pin terminal. This example is advantageous in that processing steps can be saved but a disadvantage is the lack of freedom of design because of the requirement that the number of coil patterns and the thickness of copper foils must be the same as those of the external conductor.